1. Field of the Invention
This invention relates to a frequency processing method for a radiation image to be used in making a medical diagnosis, or the like. This invention particularly relates to a frequency processing method for a radiation image, which is carried out in order that a visible radiation image having good image quality can be reproduced and used as an effective tool in, particularly, the accurate and efficient diagnosis of an illness, and with which an artifact occurring at peripheral regions in a reproduced visible image can be reduced.
2. Description of the Prior Art
Frequency processing is carried out in order that a visible radiation image having good image quality can be reproduced and used as an effective tool in, particularly, the accurate and efficient diagnosis of an illness. One example of the frequency processing is frequency emphasis processing, such as unsharp mask processing disclosed in, for example, U.S. Pat. Nos. 4,315,318 and 4,317,179. With the frequency emphasis processing, an unsharp mask signal Dus is subtracted from a read-out image signal Dorg, which has been detected from a radiation image. The obtained difference value is multiplied by an emphasis coefficient .beta.. The resulting product is added to the read-out image signal Dorg. In this manner, predetermined spatial frequency components in the image can be emphasized. The frequency emphasis processing is represented by the formula EQU D=Dorg+.beta.(Dorg-Dus)
wherein D represents the signal obtained from the frequency processing, Dorg represents the read-out image signal, Dus represents the unsharp mask signal, and .beta. represents the emphasis coefficient.
As described above, the frequency processing is carried out in order that a visible radiation image having good image quality can be reproduced and used as an effective tool in, particularly, the accurate and efficient diagnosis of an illness. However, the conventional frequency processing has the drawbacks described below. Specifically, FIG. 1 shows an example of the distribution of an original image signal. However, a radiation image signal having been obtained by two-dimensionally scanning a recording medium, on which a radiation image has been recorded, with reading light has the drawbacks in that, as illustrated in FIG. 2 and FIG. 3, a perceptible artifact 3 occurs at peripheral regions in a radiation image 2 recorded on a recording medium 1, which peripheral regions are located in the vicinity of the ends of the radiation image 2. The artifact 3 adversely affects the medical diagnosis, or the like.
The value of the emphasis coefficient .beta. used during the frequency processing is automatically set in accordance with which portion of an object is represented by the recorded image, which mode was used when the image was recorded (e.g., a contrasted image recording mode or a tomographic image recording mode), or the like. Alternatively, the value of the emphasis coefficient .beta. is manually set from an external input device. However, in cases where the emphasis coefficient .beta. is merely set automatically or manually, the artifact described above cannot be eliminated. Therefore, a need exists for an improved frequency processing method for a radiation image.